The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.
Joint replacement therapy remains the only treatment available for relieving the pain and suffering in advanced degenerative bone disease. However, the technologies available in this area of orthopaedics are far from satisfactory. For example, Australians require more than 60,000 hip and knee replacement operations annually, a rate that has been estimated to be increasing by some 10% per annum, and a staggering 25% of which are revisions of failed implants [Graves, S. E., et al., The Australian Orthopaedic Association National Joint Replacement Registry. Med. J. Aust., 2004; 180 (5 Suppl.): p. S31-4]. Further complications arise in situations where bone stock is compromised, or where initial implant stability is questionable (e.g. elderly patients, post-traumatic injuries or in revision operations), in which cases short- and long-term clinical results are typically inferior. The increases in life expectancy, and in the number of younger patients requiring implants, highlights the need for greater implant longevity and has driven biomedical research to develop novel micro-engineered surfaces to anchor the cementless prosthesis directly to the living bone through osseo-integration, thereby attempting to provide a stable interface strong enough to support life-long functional loading. It is clear that there is a serious problem with the longevity of current orthopaedic devices; a problem that is anticipated to only increase with the increasing demand from the ageing population requiring such treatments. It is clear that any improvement that could be made to increase the performance of these orthopaedics devices would be welcomed, not only by the orthopaedic community but also by the patients themselves.
Over the last century, various ceramics have been investigated for the purpose of encouraging or stimulating bone growth. For example, in the 1880's calcium sulphate (plaster of Paris) was utilised, however calcium sulphate displays a relatively low bioactivity and a relatively high rate of degradation (Tay et al., Orthop. Clin. North Am., 1999, 30:615-23). In the 1950's hydroxyapatite was utilised, however hydroxyapatite suffers from a relatively low degradation rate and poor mechanical properties (Wiltfang J., et al J. Biomed. Mater. Res. 2002; 63:115-21). In the 1970's Bioglass® was developed, however, this material is relatively hard to handle due to its inherent brittleness and has a relatively low bending strength (Cordioli G., Clin. Oral Implants Res. 2001, 13:655-65). In the 1990's calcium silicate ceramics began to be used for stimulating bone growth. However these materials display relatively high degradation rates and high pH's in vivo, which tends to retard cell growth and affects osseointegration ability. Whilst other more recent ceramics such as HAp, Bioverit®, Ceraverit® and other calcium silicates have been found to bond to living bone and meet wide clinical applications, i.e. good bioactivity, they cannot be used in highly loaded areas, such as the cortical bone found in, for example, legs, due to the relative brittleness of these materials. For at least this reason such materials typically find their use as coatings on metallic implants.
Bone, as a living tissue, has the ability to heal itself, however in some cases damage to the bone from whatever cause is too severe to allow natural healing to take place, and so a bone graft is required to stimulate regeneration. There are three main types of bone grafts: autografts, allografts and synthetic grafts. Significant research is being conducted in the field of synthetic grafts as bone substitutes since synthetic grafts can ameliorate many of the problems associated with autografts and allografts, such as limited supply, donor site pain, and immunogenicity issues.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the above mentioned prior art, or to provide a useful alternative.